The present invention relates in general to non-volatile memory devices, including phase change memories.
A phase change memory (PCM) device, also known as Ovonics Unified Memory (OUM) device, is a memory device exploiting memory cells based on a class of materials which have the property of changing between two phases, namely from an amorphous, disorderly phase to a crystalline or polycrystalline, orderly phase, which have distinct electrical characteristics, namely considerably different values of resistivity.
A non-volatile memory device includes a memory array formed by a number of memory cells arranged in rows and columns; word lines connecting first terminals of memory cells arranged in the same row; and bit lines connecting second terminals of memory cells arranged in the same column.
Individual rows of the memory array are addressed by a row decoder which receives an encoded address and biases the word line of the row being addressed at a stable and precise voltage, the value thereof depending upon the operation to be performed (read, write, verify, erase), while individual columns of the memory array are addressed by a column decoder which receives the encoded address and biases the bit line of the column being addressed at a stable and precise voltage, the value thereof depending upon the operation to be performed (read, write, verify, erase).
Non-volatile memory devices are typically of a so-called single supply voltage type. Namely, they receive a single external supply voltage. Voltages having higher values than the external supply voltage, required in the various operations performed on the memory cells (read, program, verify, erase), are supplied externally as additional voltages to the chip or generated inside the non-volatile memory device by one or more voltage elevator circuits, generally known as “voltage boosters” or, more commonly, “charge pumps.”
A charge pump is generally formed by a plurality of booster stages cascaded between the input and the output of the charge pump and each formed by a boost capacitor with a high capacitance and by a switch which is closed or opened alternately with the adjacent switch and is made by means of diodes (for example ones made using MOS transistors having gate and drain terminals connected together), or else by means of MOS transistors.
Transfer of charge from one booster stage to the next, towards the output is upon command of complementary phase signals, either overlapping or non-overlapping, supplied to the booster stages and generated by a phase-generator circuit, e.g., a ring oscillator, supplying a clock signal having a pre-set frequency and preferably of a non-overlapping-signal generator receiving the clock signal and supplying a first logic phase signal and a second logic phase signal, both supplied to the booster stages.
The boosted voltages supplied by charge pumps are, however, generally far from stable, and consequently are regulated and stabilized by voltage regulators each arranged downstream of a respective charge pump, preferably by reducing the frequency or halting oscillation once adequate pump voltage is achieved.
Non-volatile memories may have increased power consumption due to leakage by non-addressed cells.
Thus, there is a need for alternate memory designs.